The present invention relates to a multiple coil eddy current probe equipped with a coil balancing device. As a non-limitative example, the invention relates to the problem of determining, with the aid of juxtaposed coils arranged in rows or matrixes, the profile of the joint separating two parts to be welded, in order to control the automatic positioning of a member, such as a welding torch relative to the joint or vary certain welding parameters as a function of the detected profile. However, this application is not limitative and the invention can also be used for determining the profile of all conductive metal surfaces or for carrying out the non-destructive inspection of such surfaces. It can also apply to non-juxtaposed coils for supplying comparable information.
The principle of an eddy current probe consists of supplying a coil with a high frequency sinusoidal electric current, so as to produce an alternating field which induces, in an electricity-conducting member facing the probe, eddy currents producing during their return path a field which opposes the initial field and modifies the impedence of the coil. The variations of the impedence of the coil give an indication of the arrangement of the coil with respect to the facing part and the particular profile of said part. This generally relates to information on the distance separating the coil from the part in the case of a probe used in proximity. It can involve information on the structure of the part in the case of a probe used in non-destructive inspection.
An eddy current probe can have one or more coils. In the case of a probe with a single coil, the detection of a profile of a part or a non-destructive inspection of said part requires means for carrying out mechanical scanning of the part by the coil. In the case of a multiple coil probe, this mechanical scanning is often replaced by an electronic scanning of the juxtaposed coils, which in each case supply a point of the profile to be determined. Such a device is often preferable to a device with a single coil, in which the mechanical scanning introduces vibrations into the probe, which are prejudicial to the accuracy of the measurement.
In general terms, the sensitivity of the measurement is at a maximum, when the coils are regulated to overvoltage or resonance. Thus, the eddy current probes are provided with means for regulating each coil to overvoltage.
It is well known that the sensitivity of such a probe is at its maximum when the circuit is tuned to resonance. This state of resonance is defined by the relation LC.omega..sup.2 =1 where L is the overall inductance of the circuit (mainly the coil), C is the overall capacitance of the circuit, and .omega.=F/2 .lambda., where F is the frequency of the sinusoidal signal delivered by the oscillator. It is also well known that, when a resonant circuit is tuned to resonance, the voltage across the inductance (i.e. the coil) is greater than the voltage of the signal delivered by the oscillator. So, it is said that the coil is in an overvoltage state. Therefore, as used herein, "coils regulated to overvoltage" means that the resonant circuit is tuned to resonance, in order to have the greatest sensitivity.
The most frequently used regulating means consists of using, for each coil, a variable resistance--capacitance network, connected to one end of the coil on the one hand and to earth on the other. The disadvantage of this solution is that it requires the same number of resistance--capacitance networks as there are coils, which can involve several dozen networks and the same number of controls on the probe.
The use of several juxtaposed coils, which may optionally be organized in matrix-like manner, is difficult due to the magnetic interaction between adjacent coils. Thus, the signal supplied by one coil can be affected by the supply to adjacent coils, which makes it difficult to simultaneously balance all the coils.
French Pat. No. 79 10458, filed on Apr. 25th 1979 provides a solution to this problem of supplying the coils, by proposing a sequential excitation thereof. This invention also proposes the use of the same number of processing means for the signals from the coils as there are coils, which increases the number of circuits and consequently increases the cost of the apparatus and reduces its reliability.
French Pat. No. 81 11087, filed on June 4th 1981 provides an improvement to the aforementioned patent. The coils are excited in parallel and supply signals to a multiplexer connected to a single processing means. Thus, processing takes place sequentially on each of the signals of the coils. The use of a single processing means consequently makes it possible to simplify the apparatus, but the latter is still difficult to balance, because it is necessary to adjust the variable resistance--capacitance network of each coil in such a way that each of the coils is regulated to overvoltage. The expert uses here miniaturized variable air capacitance positioned as close as possible to the coils and whose variation range is a few percent, which is not always sufficient for completely carrying out the disparities in the characteristics of the various coils which, by design, may differ by more than 10%.